In a number of applications, it is necessary to monitor liquids containing significant quantities of suspended materials, which materials may tend to separate or accumulate as a separate phase beneath a body of clearer liquid. A parameter which it is frequently desired to monitor is the level of the interface between settled material or sludge, and a supernatant liquid.
Suspended material in liquids tends to reflect or disperse acoustic energy, although the extent of this effect is frequency dependent. The effectiveness of a pulse echo ranging system in determining the position of an interface between a sludge layer and a supernatant liquid will depend upon the presence of suspended material, including gas bubbles, in the supernatant layer, and the concentration and nature of particles in the sludge layer. It is difficult to select a transducer frequency which will provide reliable detection of the interface under all circumstances.
Since transducers used in such systems will normally operate submerged, there is the possibility of build-up of material passing out of suspension and liquid and building up on the transducer assembly. Such build-up can be inhibited by ultrasonic insonification of vulnerable surfaces, particularly the radiating surface of the transducer assembly, but in practice, the most effective frequencies for operation of the transducer, if it is to be self-cleaning, often fall in a different range from those providing best detection of the sludge/liquid interface.
UK Patent Application 2054851A discloses a sonar system using multiple frequencies, one of which is generated by the beating of two closely spaced higher frequencies. UK Patent Application 2191055A dicloes a radar system operating at two substantially different frequencies in order to detect flocks of birds.